Firstly, this is one of the best links for a starting Warmahordes player. It gives you a good look at the models available for each faction, and summaries of what their units do/synergize with. The summaries vary in quality and clarity, but its a good place to start: http://battlecollege.wikispaces.com/

There's an app called ibodger. It's army building software for the iphone/ipod and it's excellent and free. Great for fiddling with lists, and it offers a link to battlecollege for each unit choice if you're not sure what something is/does.

Faction summaries (mind you, this is from a newb, but I think that's probably beneficial in this case). As far as power level all of the main factions are very competitive. The only one I've heard some negatives about are Retribution, and they're the new kids on the block. Mind you each faction plays somewhat differently depending on the caster your select:

Cryx - Cryx have some of the best offensive units in the game. They're very tricky, emphasizing arch nodes (basically jacks you can cast spells through) and debuffs. They have probably the best jack in the game -- the Deathjack. They do stuff like gather souls from living models for more focus, resurrect their own models, etc. Thinking of black in magic the gathering is apt. Their casters feats do stuff like freeze the enemy for a round, or res troops behind the enemy units, or create a zone of debuff.

Menoth - Kind of the opposite of Cryx. Instead of debuffing the enemy, they emphasize buffing their own units. Their jacks are statistically mediocre but cheap, until they're buffed up when they become superb. They come with a lot of immunities to powers and effects -- its basically a counter army (think white/blue in magic). Range is generally limited, these guys generally slog it, weather the storm, and grind the game out. Lots of zone dispel or conflagarations for caster feats.

Cygnar - Emphasis is on ranged combat. Pricey jacks with a lot of interesting utility. Cool effects like arching lightning.

Khador - Beefy jacks that are the toughest in the game, point-for-point. They also hit very hard and have a solid ranged game.

Also Pete depending how you veer playstyle wise You can probably pick up a decently interchangeable list with 2 casters for about $200 not counting books and such. Less depending on if the starter boxes have a good amount of the stuff you need for your particular dude.

Lol, @ easiest to play... not this again. As of right now, there are two regular Cryx players at the store and a few people who have them along with 1-2 other factions (sometimes 3 other factions). Honestly, choose your army off of which models you like the most. I was stuck between Legion and Cryx in terms of models and ended up likening Cryx better. Once you find models you like, rules will follow.

Loyal followers of the Creator of Man, Menites work together to overcome the toughest opponents, letting nothing stand in their way. While sometimes individually weaker, more than any other faction they leverage synergy to combine into an unstoppable force. Every Menite killed in battle just inspires his fellows to fight even harder until all the Iron Kingdoms submit to Menoth's Law.

Menoth Melee: Blessed with cheap, effective troops, the Protectorate has a mix of fast precision troops and slower hard-hitting Knights. These units are supported by a combination of commanding officers who ensure their faith does not waiver in the face of death.

Menoth Ranged: Preferring quantity over quality, there is a strong emphasis on AOE and spray attacks over the more accurate direct-fire weapons of other factions. Menites do not need to hit in order to kill, instead letting their wandering blasts land where they may, causing unquenchable fires in many cases. Aiming is even less necessary if your fanatical troops are willing to put themselves in harm's way to fire from point-blank distance.

Menoth Magic: Menoth spells focus on protection and enhancement of their army. They protect against magic, shield their troops, and inflict divine punishment on anyone who attacks them. Offensively their blessings inspire their troops to fight better while raining holy fire upon the enemy from afar. Some Menoth warcasters can even harvest the souls of their dying troops to inflict one last attack upon the enemy.

Menoth Warcasters: Menite warcasters excel in helping their army do more. Most of them have wide-ranging spells that help buff their army up to superhuman levels. While blasting the enemy with cleansing fire, they up the accuracy and damage output of their own troops and warjacks to high levels.

Menoth Warjacks: Despite early problems in acquiring the knowledge to manufacture warjacks, the Protectorate now has an arsenal of warmachines that rival any nation of Immoren. They serve roles from anti-infantry and anti-warjack, to durable arcnodes, to warcaster bodyguards. While slightly less armored than 'jacks of Khador, and somewhat less agile than Cygnaran designs, Protectorate warjacks strike a balance and are supported like no other army's can be - through the prayers and hymns of the faithful. With this support they often arrive at their chosen targets unscathed.

Cryx Primary thanks to Aldarionn

The Nightmare Kingdom of Cryx has long been the "bogeymen" that other kingdom's mothers tell their children about to scare them to eat their broccoli and do their chores. The only problem is that the "bogeymen" are real. Very real. Undead horrors abound in Cryx. But these aren't your shambling, mindless undead. They run screaming at you at a dead sprint, bringing swift death... and the eventual rebirth as yet another in the ranks of the always-growing undead masses.

Cryx Melee: Cryx excells in melee due to their speed and generally stealthy nature. Able to conduct lightning strikes before the enemy has a chance to get a bead on them, they rush in and can pound you to death before you even realized they were coming. Generally, their melee attacks can be augmented in some way, either through combo-strikes or focus-feedback or magical manipulation. Just "punching you" rarely happens in Cryx.

Cryx Ranged: Modestly sparse in the faction, Cryx ranged attacks, like their melee, tend to have something else along for the ride. They may cause your trooper to explode, or force them to choose between attacking or moving next turn, or some other form of nastiness. Though this isn't where Cryx is going to be winning most of their games, their ranged effectiveness is a set-up to the melee waiting to come.

Cryx Magic: Cryx loves to see their enemies disolve into puddles of goo, and as such, have numerous spells that cause acid damage to their opponents. Their highly-accurate magic attacks help out wheir their lack of ranged attacks may turn into a problem. With the most arc nodes available, their warcaster's spells can come from almost any angle. Other than that, Cryx is master of debuff. Messing with your opponent's armor or defense values only makes their, what may seem like modest, attack ability look over-the-top.

Cryx Warcasters: Cryx, like most other factions, has a host of vastly different Warcasters who all work differently depending on the play style of the general fielding them. We have strong, direct assassins like Warwitch Deneghra and Skarre, Queen of the Broken Coast, and murderous meat grinders like Iron Lich Asphyxious and Lich Lord Terminus. While some Warcasters may have similarities, or share similar spells, no two play the same, and swapping out Skarre for Asphyxious within the exact same force will give it a totally different focus.

Cryx Warjacks: Cryxian Helljacks are some of the fastest Warjacks in the Iron Kingdoms, and although they have lighter armor than most of the rest, they pack a serious punch. We also employ more Arcnodes than any other faction, and Bonejacks such as the Deathripper, the Nightwretch and the Defiler are seen in numbers on the battlefield. They are blazing fast vessels capable of providing our Warcasters a safe and easy way to launch punishing spells into our opponents back ranks, and solidifying our position as the most magically dominant force in Western Immoren.

Honestly, choose your army off of which models you like the most. I was stuck between Legion and Cryx in terms of models and ended up likening Cryx better. Once you find models you like, rules will follow.